Narrow weld-groove welding process and apparatus therefor

ABSTRACT

A narrow weld-groove welding process, in which a bare welding wire is fed into a weld groove defined between the opposed surfaces of two pieces of metals for producing a metal arc therein for welding. In this process, a welding wire is subjected to a plastic deformation of a wave form, before being fed into a nozzle hole provided in a contact tip, and then the wire is fed into a nozzle hole, while maintaining elasticity tending to cause waving, whereby the tip of a welding wire being fed through a nozzle exit is automatically waved between the opposed surfaces of metals to be joined, with the tip of wire being alternately faced in the opposite directions, in response to the weaving motion thereof, with the feeding of the welding wire and progress of welding. In addition, the apparatus for use in a narrow weld-groove welding process is also disclosed.

This is a division of application Ser. No. 776,921, filed Mar. 11, 1977,now U.S. Pat. No. 4,188,526, issued Feb. 12, 1980.

This invention relates to a narrow weld-groove welding process and anapparatus therefor, in which two pieces of metals are placed in opposedrelation so as to define a weld groove therebetween, and a bare weldingwire is fed into the weld groove from a contact tip provided at the tipof a contact tube, thereby producing a metal arc in a shield gas or anatmosphere of a flux for depositing a welding wire on the surfaces ofmetals to be joined.

Hithereto, there has been proposed an attempt in a MIG (Metal Arc InertGas) welding process for thick steel plates, with a narrow weld groovedefined between the opposed surfaces thereof, in which process a weldingwire of a small diameter is imparted elasticity tending to causebending, and then the wall surface of a weld groove on one side iswelded, followed by welding on the other. However, this attempt suffersfrom disadvantages in that a contact tip of a contact tube inserted intoa weld groove is liable to consume excessively; an arc produced duringthe welding of a wall of a weld groove on one side is free from weaving,thus resulting in lack of penetration or high temperature cracking; andundesired stability or consistency in feeding a welding wire results. Incontrast thereto, there has been proposed an attempt (the JapanesePatent Publication No. Sho50-20031) in which the tip of a welding wireis oscillated or weaved. However, this latter attempt is not a rightsolution to this problem, because there should be provided a mechanismfor weaving a contact tube in a weld groove in its entirety, and thusthis attempt is not suitable for a narrow weld-groove welding. A stillanother attempt is disclosed in the Japanese Patent Publication No.Sho49-17946, in which a welding wire is imparted an elasticity tendingto cause bending in a given direction beforehand, and then the weldingwire is wound aroung a rocking plate, while the rocking plate is rockedand rotated through a given angle about a feeding axis of a weldingwire, so that the tip of the welding wire being fed into the weld groovemay be weaved. However, this attempt poses still another problem that abending and feeding mechanism for a welding wire is large in size, andthe undesireable consistency in feeding a welding wire results becausevarious mechanisms have to be inserted or incorporated in a range from awelding wire source to an arc point, and the direction of welding isirreversible due to the exsisting of it is wire bending habit.

The following are various gas shielding processes employable in a narrowweld-groove MIG welding process.

(a) A gas nozzle is placed outside of a weld groove, so that a shieldgas is injected through a gas nozzle towards a welding zone, therebyshielding atmosphere.

(b) A gas nozzle surrounding a contact tube is inserted into a weldgroove for shielding a weld zone from atmosphere.

(c) Gas is injected from a side hole in a gas nozzle, in addition to theinjection of gas through a nozzle tip, as in a manner disclosed in theparagraph (b).

(d) A first shield gas is injected towards a weld zone through a passagerunning in parallel with a guide path for guiding a welding wire, whilea second shield gas is injected from a surface outside of the weldgroove.

However, according to processes (a) and (b), a shield gas is injectedand supplied from a surface outside of a weld groove, so that alimitation is imposed on a thickness of metals to be joined. Accordingto the processes (b) and (c), a gas nozzle surrounding a welding wire isinserted into a weld groove, so that a limitation is imposed on thewidth of a weld groove, when the width is desired to be reduced.

Hitherto, several welding processes of this type are known in Japan.(One of example is the Battelle process developed in the U.S. of theBattelle Memorial Institute). A majority of the welding processes ofthis type use a contact tube adapted to guide a welding wire into awelding groove, except for high current MIG welding processes using awelding wire of a large diameter. The diameter of a welding wire used ison the order of 1 mm, and a welding current which approximates acritical value required for spray transfer of molten metal droplets isselected, with the result that there is necessarily produced spatters(This term is defined as slag or molten droplets splashed from an arc orcrater.), so the spatters thus produced cling to the tip of a contacttube or an injection exit of a nozzle, thereby exerting adverse effecton welding characteristics. In addition, a welding current to be used islimited to a value approximating a critical current, so that theresulting weld droplet-transferring mode is not suited for all positionwelding.

In addition, MIG welding and TIG (tungsten inset gas) welding processes,when used for a narrow weld-groove welding, suffer from defectives asare incurred to the other welding processes, for instance, lack offushion into a base metal, blow holes, slag inclusion, lack ofpenetration or fusion in a preceding layer or underlayer of beads.

OBJECT OF THE INVENTION

It is an object of the present invention to provide a narrow weld-groovewelding process which avoids the disadvantages confronted by the priorart welding processes.

It is another object of the present invention to provide a narrowweld-groove welding process, which may prevent spatters, and allows awelding position of any type.

It is a still further object of the present invention to provide anarrow weld-groove MIG welding process which may positively effectivelyshield a weld zone from atmosphere.

It is a yet further object of the present invention to provide a narrowweld-groove welding process and an apparatus therefor, in which inertgas is used not only for the purpose of shielding a welding arc fromatmosphere but also for the purpose of changing a contour of a moltenpool intentionally, thereby preventing defective welds, when resortingto MIG and TIG narrow weld-groove welding processes.

It is a further object of the present invention to provide an apparatusfor use in a narrow weld-groove welding process, providing a weldingwire passage, along which a welding wire is fed in the directionsubstantially in parallel with an axis of wire draw rollers, andfurthermore a welding wire may be fed in the direction perpendicular toa plane including two axes of draw rollers.

FIG. 1 is a perspective view of an outline of the process and apparatustherefor, according to the present invention;

FIG. 2 (A) is a longitudinal cross-sectional view of a contact tube foruse in the present invention, FIG. 2-B is a cross-sectional view takenalong the line B--B of FIG. 2-A;

FIGS. 3-A to 3-C are views illustrative of the weaving locuses of anarc, shown in cross section taken along the line B--B of FIG. 2;

FIG. 4 is a longitudinal cross-sectional view showing a nozzle for usein a narrow weld-groove MIG welding process according to the presentinvention;

FIG. 5 is a cross-sectional view taken along the line V--V of FIG. 4;

FIG. 6 is a perspective view showing the entire arrangement of anapparatus embodying the present invention;

FIG. 7 is a cross-sectional view, partly broken, of a mechanism forimparting a welding wire elasticity tending to cause bending;

FIGS. 8A and B are cross-sectional views taken along the line VIII--VIIIof FIG. 7;

FIG. 9 is a perspective view showing the construction of a narrowweld-groove welding apparatus according to the present invention;

FIG. 10 is a cross-sectional view showing a welding nozzle portion, inaddition to the formation of a wavy wire and a displacement of the tipof the wire;

FIG. 11 and FIG. 12 are cross-sectional views showing the positionalrelationship between a weld bead and nozzle openings, taken along theline XI--XI of FIG. 10;

FIG. 13 is an enlarged side view of an arcuate tube in a welding wiresupply means according to the present invention;

FIG. 14 is an explanatory view of a force acting on a weld droplet;

FIG. 15 is an explanatory view of a weld droplet;

FIGS. 16A to 16F are explanatory views showing a transient phase of welddroplet from the tip of a wire to a base metal;

FIGS. 17A to D are explanatory views showing a wave form of pulsecurrent, electromagnetic pinch force, and a separating condition of amolten metal;

FIG. 18 is a diagram of one embodiment of a power source circuit for usein the present invention;

FIG. 19 is a diagram showing another embodiment of a power sourcecircuit for use in the present invention; and

FIGS. 20A to 20C are transverse cross-sectional views illustrative ofwelds produced by short-circuiting arc, pulse arc and spray arc;

FIG. 21 is a longitudinal cross-sectional view showing a weldingapparatus and beads produced in a narrow weld-groove welding process andapparatus therefor according to the prior art;

FIG. 22 is a transverse cross-sectional view of the bead;

FIG. 23 is a cross-sectional view of the apparatus according to thepresent invention, taken along the line XXIII--XXIII of FIG. 24;

FIG. 24 is a longitudinal cross-sectional view of the apparatusaccording to the present invention, taken along the line XXIV--XXIV ofFIG. 23;

FIGS. 25, 26, 27 are transverse cross-sectional views of primary orsecondary gas nozzle, showing various shapes of gas passages;

FIG. 28 is a transverse cross-sectional view of a bead showingpenetration of a weld according to the welding apparatus of theinvention; and

FIG. 29 is a longitudinal cross-sectional view showing one example, inwhich the injecting directions of gases through two or more gas passagesare varied.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a perspective view showing the entire arrangement of oneembodiment of an apparatus according to the present invention. Referringto FIG. 1, shown at 1 is a welding wire, at 2 straightening rollers, at3 first feeding rollers, at 4 a conduit, at 5 a welding-wire bendingguide tube, at 6 a rocking plate, at 7 a rocking motor, at 8 a rollerdrive motor, at 9 a second feeding rollers, and at 10 a contact tube.The welding wire 1 is staightened by means of the straightening rollers2 into a linear form, and then delivered from the first feeding rollers3. The welding wire 1 thus delivered from the first feeding rollers 3 isguided through the conduit 4 and directed into the welding-wire bendingguide tube 5. The guide tube 5 runs along the outer periphery of therocking plate 6 up to the entrance of the second feeding rollers 9. Thewelding wire 1 thus guided through the guide tube 5 is supplied throughthe feeding rollers 9 into a nozzle hole provided in the contact tube 10to be described later. Shown at 8 is a roller drive motor adapted todrive the second feeding rollers 9, and at 7 is a rocking motor adaptedto rock the rocking plate 6. When the rocking plate 6 is rocked aboutits one end in the direction at a right angle to the surface of thefigure, the welding wire 1 is imparted a wavy plastic deformation due toa combination of the downward feeding of the wire by means of the secondfeeding rollers 9 positioned immediately before or downstream of thecontact tube 10a with the rocking motion of the rocking plate 6.

FIG. 2 shows a detailed construction of a contact tube for use in thepresent invention. FIG. 2 (A) is a cross-sectional view thereof and FIG.2 (B) is a cross-sectional view taken along the line IIB--IIB. In FIG.2, shown at 1 is a welding wire, at 5 a welding wire bending guide tube,at 9 feeding rollers, at 10a a contact tube, at 10b a contact tip, at11, 11' metals to be joined, at 12 a backing material, at 16 a nozzlehole of an elongated cross section, at 17 an injection hole throughwhich to inject an inert gas such as argon towards a weld groove, and at18 water passages for circulating cooling water therethrough.

The nozzle hole 16 is of an elongated cross section in the direction ata right angle to the opposed walls or surfaces of metals 11, 11'. Thewelding wire 1 is fed through the nozzle hole 16, while maintaining wavyplastic deformation, from the side of the feeding rollers 9 towards thebacking material 12. The welding wire 1 which has been fed into a weldgroove through the lower opening of a nozzle hole 16 tends to restore aplastic deformation imparted by the feeding rollers according toelasticity of the welding wire along the width of the weld groove, asshown in FIG. 2 (A). The amplitude of weaving of the tip of a weldingwire along the width of the weld groove is dependent on a rocking angleof the rocking plate as well as an extension of the welding wire (lengthof welding wire protruding from the nozzle tip). This permits sufficientpenetration of a weld on a corner formed by the backing material 12 andopposed surfaces of metals to be joined. With the progress of welding,an arc produced at the tip of a welding wire automatically reciprocatesor weaves between the opposed surfaces of the metals 11, 11'.

FIG. 3 is a cross-sectional view taken along the line IIB--IIB of FIG.2-A, showing the weaving locus or pattern of an arc in this case. Shownat 13 in FIG. 3 (A) is an arc locus in case the nozzle hole 16 is of enelongated cross section in the direction at a right angle to the opposedsurfaces of metals 11, 11', as shown in FIG. 2. An arrow mark shows theadvancing direction of welding. Shown at 14 in FIG. 3 (B) is an arclocus, in case the center line of an elongated cross section of thenozzle hole 16 is inclined to the direction at a right angle to theopposed surfaces of metals 11, 11', in which the arc locus is notsymmetric with respect to the advancing direction of welding. However,in this case, as well, there may be achieved a stable arc locus, andthus this version of welding process may be used as required, with theachievement of deposited metals of a high quality. Shown at 15 in FIG. 3(C) is an arc locus resulting, in case the center line of an elongatedcross section of the nozzle hole 16 runs in the direction at a rightangle to the opposed surfaces of metals 11, 11' as in the case of FIG. 3(A), and yet the rotational speed of the rocking motor 7 is socontrolled as to be stopped intermittently, thereby forming arectangular wave form of an arc locus. As a result, the dwelling time ofan arc on the opposed surfaces of metals 11, 11' may be extended,thereby insuring sufficient penetration of welds on corner portions. Inaddition, the procedures (B) and (C) may be combined in use. Theadjustment of dwelling time of an arc may be controlled by changing amode of speed control of the rocking motor 7. As is apparent, theposition of an arc may be automatically weaved between the opposedsurfaces of metals, with the progress of welding and feeding of awelding wire, and in addition the dwelling time of an arc on the opposedsurfaces of metals may be adjusted as desired, so that the weldingaccording to the present invention permits efficient welding andprovides a wide application. In other words, all position welding may beapplied to pipes of a large thickness and diameter, as well as to thebuild-up welding other than the narrow weld-groove welding.

The following numerical values are given as examples of the narrowweld-groove welding according to the present invention:

thickness of metals to be joined: 150 mm

length of weld groove in the welding direction: 1000 mm

width of weld groove: 10 mm

arc locus: FIG. 3 (a)

weaving width or amplitude: about 7 mm

Inert gas: Ar mixed CO₂

flow rate of gas:

Ar: 40 litr/min,

CO₂ : 10 litr/min

welding speed: 300 mm/min

In the above welding condition, there may be obtained a weld of aquality acceptable for JIS--one grade.

As is apparent from the foregoing description of the present invention,a welding wire is imparted a wavy plastic deformation, immediatelybefore being fed into a contact tube, and then the wire is fed throughthe nozzle hole of an elongated cross section, while maintaining a waveform, thereby enabling a weaving action of a welding wire within a weldgroove, so that an arc may be automatically weaved between the opposedsurfaces of metals. As a result, a consistent feeding of welding wirebecomes possible, even in case the thickness of metals are increased,and in addition the weaving direction of an arc may be maintainedconstant. In addition, the weaving direction of an arc may be changed tomeet an intended application, thus allowing complete penetration of aweld on the walls of a weld groove as well as welding in any position.Meanwhile, description has been had to the MIG welding process. However,the present invention is by no means limited to this instance but may beapplied to submerged arc welding and the like. In short, the presentinvention may be applied with the same result to any type of welding, asfar as a bare welding wire is used for a narrow weld-groove welding.

Embodiments of nozzles for use in narrow weld-groove MIG welding will bedescribed with reference to FIGS. 4 and 5.

Referring to FIGS. 4 and 5, shown at 21 is a contact tube made of acopper alloy, an outer diameter and inner diameter thereof being 5 mm,and 3 mm, respectively. Shown at 22 is a cooling water feeding tube and,at 23 a cooling water return tube. Accordingly, cooling water iscirculated in arrow directions through the tubes 22, 23 for cooling anozzle and improving electric conductivity thereof, in addition toprevention of burning of a heat resisting, electrically insulatingmaterial 30 to be described hereinafter. Shown at 24 is a shield gassupply tube, and thus a gas containing 80% argon gas, and 20% carbondioxide gas may be directed to a weld groove portion for injectiontowards a deposited metal. Shown at 25, 26 are impure-gas suction anddischarge tubes for introducing and discharging impure gases into andfrom a weld groove. A vacuum is maintained in the impure gas suction anddischarge tubes 25, 26 for introducing impure gases under suction, sothat even a shield gas injected through a nozzle exit of a smalldiameter may be well spread around a weld at a low flow speed, with theaid of the aforesaid suction, thereby protecting a weld, withoutexerting an adverse effect on a deposited metal. The tubes 21 to 26 arearranged flatwise, and brazing is applied to respective tubes to bejoined together, after which the outer surfaces of the tubes are coveredwith a heat resisting, electrically insulating material 30, such asglass fibers or the like. The diameters of the respective tubes 21 to 26are so selected as to be equal to the outer diameter of the contact tube21. Shown at 27 is a weld bead. In the embodiment, the width of a weldgroove is selected as being 8 mm. Shown at 28 is a metal to be welded.Shown at 1 is a welding wire of 1.2 mm in diameter. Although not shown,the welding wire 1 is imparted a wavy plastic deformation, immediatelybefore being fed into the contact tube 21, thus allowing a weavingmotion of the tip of a welding wire according to elasticity thereof,after being fed into a weld groove. This insures sufficient penetrationof a weld into a base metal.

As can be seen from the foregoing, there are arranged flatwise in sideby side relation the contact tube 21 adapted to feed a welding wire intoa weld groove, the cooling water feeding tube 22 and its return tube 23which are positioned sidewise thereof, shield gas feeding tube 24, andimpure gas suction and discharge tube 25, and another suction anddischarge tube 26, which are positioned on the opposite sides of thetubes 1, 22, 23, respectively. As a result, the width of a weld groovemay be reduced to a considerable extent. In addition, impure gasesaround a weld portion may be introduced under suction and dischargedoutside, so that despite shield gas being injected through a nozzle exitof a small diameter, the gas may be directed towards the bottom of aweld groove following a satisfactory flow pattern. This improves thepurity of a shield gas, and the shield gas may be fed only through aweld groove towards its bottom. In addition, gas suction and dischargeopenings are provided adjacent to the bottom of a weld groove, so thatno limitation arises for the thickness of metals to be joined, whilepermitting safisfactory feeding of a shield gas. Furthermore, since afume or smoke produced during welding may be discharged, observation andmonitering of an arc may be attained with ease.

As a welding power source for supplying a current to the welding wire 1,there has been proposed a pulsed arc welder not shown, which may supplya pulse wave current of 120 Hz. A combination of the aforesaid powersource with a welding nozzle according to the present invention permitsthe complete transferring of a droplet in a relatively low currentrange, thereby avoding spatters, with the result of freedom of adhesionof spatters to a contact tip and a shield-gas exit of a nozzle, unlikethe prior art welding apparatus.

In this embodiment, a weld zone in the bottom portion of a weld groovealone is shielded, and tubes are joined flatwise, so that a narrowweld-groove welding is enabled for metals having an increased thickness,thus providing a weld zone of a consistent quality with the accompanyingmany industrial advantages.

Description will now be given of the second embodiment of the narrowweld-groove MIG welding process according to the present invention.

The feature of this embodiment is that a welding wire is imparted a wavyplastic deformation, immediately before being fed into a nozzle hole ofan elongated cross section, and then the wire is fed into the nozzlehole while maintaining elasticity tending to cause waving, so that thetip of a welding wire being fed through the tip of a nozzle is weavedautomatically, with the direction of the tip being alternately changedin opposite directions, in response to the weaving motion thereof.

FIG. 6 shows an entire arrangement of the apparatus embodying thepresent invention.

FIG. 7 shows, in cross section, a mechanism for providing a welding wirean elasticity tending to cause bending or curl. Referring to FIGS. 6 and7, shown at 41 is a rocking nozzle, at 42 a drive source for the rockingnozzle, i.e., a rocking motor, at 43 a worm gear adapted to rotate,along with a rotary shaft of the rocking motor 42, at 44 a rocking shaftadapted to rotate in response to the rotation of the worm gear 43 andcoupled to the worm gear 43, at 45 a welding-wire feeding motor servingas a drive source for the feeding rollers 46, at 47 press rollersadapted to rotate in response to the rotation of the feeding rollers 46in contact with the outer peripheries of the feeding rollers, at 48 acontact tube adapted to introduce a welding wire 1 into its nozzle hole,which wire is being fed from the feeding rollers and press rollers 47,and at 49 metals to be joined.

The welding wire 1 is supplied from above the rocking nozzle 41, whilethe rocking nozzle 41 effects a rocking motion following a sectorpattern in a plane as shown in FIG. 7, through the medium of worm gear43 and rocking shaft 44, according to the cyclic normal and reverserotations of the rocking motor 42. This imparts a wavy deformation tothe welding wire 1. The welding wire 1 which has passed through therocking nozzle 41 is then passed through the feeding rollers 46 and thepress rollers 47, wherein the wire is imparted a downward force andforced into a nozzle hole in the contact tube 48 positioned immediatelydownstream of rollers 46, 47 and then the wire 1 is moved through thenozzle hole, while maintaining elasticity to cause waving, and then intothe weld groove. The welding wire which has entered the weld grooverestores its wavy deformation according to its elasticity. In thisrespect, the nozzle hole provided in the contact tube 48, as shown at 52in FIG. 8, is of an elongated circular cross section, so that theweaving motion of the weld wire is restricted by a major axis of theelongated circular cross section.

FIG. 8 is a cross-sectional view taken along the line VIII--VIII of FIG.7. FIG. 8 (A) refers to a case where the directon of a major axis of anelongated circular cross section is at a right angle to the width of aweld groove, while FIG. 8 (B) refers to a case where the direction of amajor axis is slant to the width of the weld groove. Shown at 40 in FIG.8 are arc locuses in the respective cases. More particularly, thewelding wire 1 which has been fed into a weld groove through a lowerexit of a nozzle hole 52 of an elongated circular cross section, whilemaintaining a wavy plastic deformation, tends to be directed along themajor axis of the elongated circular cross section according to theelasticity of the wire. In this case, the amplitude of the weaving of awelding wire is controlled by a rocking angle a of the rocking nozzleand an extension L of the welding wire from the tip of a nozzle in aweld groove. This permits sufficient penetration of a weld into thecorner portions formed between a backing material 53 and opposedsurfaces base metals 49. In addition, with the progress of welding, anarc produced at the tip 1c of a welding wire automatically weves betweenthe opposed surfaces of metals to be joined. Shown at 40 in FIG. 8 is aweaving locus of an arc in this case, while an arrow designates theadvancing direction of welding. In case the major axis of an elongatedcircular cross section of a nozzle hole 52 is slant to the verticaldirection, as shown in FIG. 8 (B), then the arc locus resulting is notsymmetric with respect to the advancing direction of welding. However, astable or consistent arc locus may be attained in this case as well, sothat a deposited metal of a good quality may be obtained according toits intended application. For instance, the arc locus shown in FIG. 8(A) is best suited for a flat position welding, vertical positionwelding and overhead position welding and the like, while the arc locusof FIG. 8 (B) is suited for a horizontal position welding.

As is apparent from the foregoing description of this embodiment, awelding wire for MIG welding process is imparted a wavy plasticdeformation, immediately before being fed into a contact tube, and thenmoved through the nozzle hole having an elongated circular crosssection, while maintaining a wavy deformation, thereby permitting theweaving motion of the tip of a welding wire within a weld groove, withthe result that an arc may be automatically weaved between the opposedwalls of the weld groove. As a result, there may be achieved stable orconsistent feeding of a welding wire into the weld groove, with theweaving direction of an arc maintained constant. In addition, theweaving direction of an arc may be changed as required, and there may beachieved sufficient penetration of a weld into the walls of a weldgroove. The aforesaid embodiment according to the present inventionsolved many problems experienced with the prior art welding processes,in which a welding wire is imparted an elasticity tending to causebending in a given direction by means of bending rollers, and then thewelding wire is wound around a rocking plate, while the rocking plate inits entirety is rocked and rotated through a given angle about thefeeding axis of a welding wire, thereby causing the tip of a weldingwire to weave within a weld groove. More specifically, the followingdisadvantages of the prior art processes may be avoided i.e., a bendingand feeding mechanism for a welding wire is complex and large in size;since various mechanisms are placed between a welding-wire-feedingsource and an arc point, there results the failure in stability infeeding of a welding wire, because of an elasticity tending to causebending in the wire in a given direction, the advancing direction ofwelding is not reversible; and the weaving of a welding wire is hindereddue to spatters clinging to the tip of a contact tube during thewelding, with the accompanying failure in weaving motion of an arcwithin a weld groove.

Description will be turned to an embodiment of a welding apparatus, inwhich the formation of a wavy welding wire and the feeding thereof areaccomplished at one time.

Referring to FIGS. 9, 10, 11 and 12, the welding wire 1 is supplied froma wire reel past a conduit 62 to a welding device 63 according to thisembodiment. In this case, there may be provided a wire feeding meanshaving a drive rollers additionally, for achieving smooth paying--out ofa wire from a wire reel. The welding device 63 consists of awelding-wire supply means 64 and roller means 65 including a weldingnozzle. These components are supported on a frame not shown. The weldingwire fed into the conduit 62 is then fed into an arcuate conduit 69running along the periphery of a frame 68 secured to a shaft 67 of theframe 66. The wire-receiving end of the arcuate conduit 69 is rotatablysupported by the frame 66, with the attaching portion thereof beingpositioned on an extension line of the shaft 67. The configuration ofthe arcuate conduit 69 is smoothly curved, so that the welding wire maybe delivered from the other end, i.e., an end 70 on the side of a nozzlesmoothly. The frame 68 swings through a given angle at a given speedabout the shaft 67, being driven by an connecting arm 72 coupled to aspeed change geared motor 71. The axis of a swinging motion of the frame68 through a swing angle α is positioned on a plane including the twoaxes of two rollers. The shaft of a drive roller of the above tworollers is connected to a reduction gear shaft within a casing of theroller means 65, which shaft is coupled to the change geared motor 75.The welding wire 1 is formed into a wavy wire 1a having a given pitchwhich depends on the swing angle and speed of the arcuate conduit 69 inthe wire supply means 64, and the r.p.m. of the roller 74 (wire feedingrate) in the roller means 65, which is associated therewith. The wavywelding wire 1a is passed through a welding nozzle 77, and one end 1bthereof extends from the contact tube hole 78 outside in a manner thewire may contact the inner wall surface of the tube 78 for supplying acurrent therethrough, thereby forming a steady arc in a weld grooveportion to provide weld beads. Thus, the wire is consumed progressively.The contact tube hole 78 should be of a cross section having a majoraxis and minor axis for accommodating the feeding of a welding wire, forinstance, elongated circle, ellipse, and rectangle. The major axisshould run at a right angle or a slant to the direction to form beads,depending on a welding condition. Two or more inert gas supply holes 79and cooling water passages 80 are provided in the welding nozzle 77adjacent to the welding nozzle hole 78. In this manner, a continuous butstable arc is produced between the end of the wavy wire 1a and a weldzone, thereby providing welds free of a defect in the metal plates 76a,76b. In short, the welding device according to this embodiment of thepresent invention features that there are provided a welding wire supplymeans adapted to swing about one axis, and a set of roller means adaptedto continuously supply a welding wire into a welding nozzle hole,thereby drawing the welding wire from the welding wire supply means intoa wavy form.

According to this embodiment, a wavy wire may be readily obtained bymeans of the welding-wire supply means which swings through a givenangle at a given speed, and two sets of roller means. As a result, thisenables the narrow weld-groove MIG welding, particularly for thickplates, without causing a defect, and with a desired rapidness andpositiveness. The two sets of roller means in this welding deviceenables the formation and drawing of a wavy wire at a time, and thewelding device is simple in construction, free of a trouble and easy inhandling. The axis 61 of the conduit 62 is coaxial with the shaft 67 ofthe swinging guide means 68. The direction of the axis 61 should besubstantially in parallel with the axes of a set of rollers 74, 76. Thisfact is essential to the effect that a welding wire drawn through therollers 74, 76 be waved and maintain its wave configuration in a commonplane accurately, and that the welding wire be smoothly introduced intothe contact tube hole 78. A low profile welding device may be provided,because of the feeding of a welding wire which runs substantially inparallel with the axes of rollers. In addition, the welding devicepermits vertical-position welding. The axis 61 should preferably bepositioned in a plane including the axes of two rollers 74, 76, inparallel with the shafts of rollers 74, 76.

In short, the following features are of importance:

(1) A process in which a welding wire is drawn by roller means andsupplied to a welding nozzle in a wave form, characterized in that thewelding wire is supplied from a direction which is substantially inparallel with the shafts of rollers, past a swing guide means to theroller means.

(2) A welding wire supply device for use in a welding device, in which awelding wire is drawn by roller means in a wave form, and supplied to awelding nozzle, characterized by a swing guide means adapted to receivea welding wire from the direction parallel with the shafts of rollers inthe aforesaid roller means and supply same to the aforesaid rollermeans.

According to this embodiment, the feeding of a welding wire my beaccomplished smoothly, and a low profile welding device may be achieved.In addition, welding positions other than flat position are enabled.Particularly, there may be achieved welds, rapidly and positively, in anarrow weld-groove MIG welding process, without a defect.

In the welding device shown in FIGS. 9, 10, 11, 12, the arcuate conduit69 running along the outer periphery of the swing guide means 68changes, through an angle of 90°, the direction of a welding wire whichhas been directed through the conduit 62 in parallel with the metalplates 76a, 76b (in the horizontal direction) and allows downwardrunning of the wire at a right angle to a plane including axes of tworollers 74. However, an excessive change in the direction of a weldingwire in an attempt to reduce the size of a welding device results indeformation in a wave form of the welding wire to be supplied to awelding nozzle, thereby hindering smooth movement of a welding wirethrough the contact tube hole 78. Accordingly, the curvature of thearcuate conduit 69 should be smooth, and in addition, the radius ofcurvature of the conduit should be such as not to cause deformationbeyond the elastic limit. The wire which is fed out of the arcuateconduit shown in FIG. 13 restores its linearity, and is then waved dueto the swinging motion of the swing guide means 68 by the cooperation ofthe rollers 74.

Because of the arcuate conduit of the aforesaid arrangement, the feedingof a welding wire may be accomplished smoothly, and there may beachieved a low profile welding device, thus enabling various weldingpositions and formation of a desired wave form for a welding wire.Particularly, there may be obtained a satisfactory narrow weld-grooveMIG welding without a defect, rapidly, positively.

Another embodiment of the narrow weld-groove MIG welding processenabling all position welding is shown in FIGS. 14 to 20, which avoidsspattering and other shortcomings in the prior art MIG welding of thistype. The feature of this embodiment lies in a pulsed-arc weldingprocess which enables the complete spray transferring of molten metaldroplets in a low current range, i.e., an arc is produced between awelding wire and metals according to a current, in which a large currentis superposed on or added to a D.C. basic current for a short timecyclically, for the narrow weld-groove welding process.

In general, in the case of arc welding, a base metal is fused due toheat from an arc, while the tip of a welding wire is also heated anddrops in a liquid form. A mode in which the molten metal produced on awelding wire is transferred to the side of a base metal is closelyrelated to the results of welding.

When a welding current falls in a low range, as shown in FIG. 14, amolten metal produced due to hear from an arc is formed at the tipthereof in a spherical form. Thus, when the weight W of a molten metalball is increased to an extent that it can no longer be supported by asurface tension πDρ (D: diameter, ρ: density) acting on a root portionof the molten metal ball, then the molten metal ball drops by itsgravity. In the actual arc welding, there is no such an idealtransferring of a droplet, i.e., the transferring due to a surfacetension and gravity alone, i.e., with the freedom of other type ofexternal forces. In the welding operation using a large current, thebehavior of a molten metal is closely governed by an electromagneticcontracting force (electromagnetic pinch force) produced by a weldingcurrent.

When a large current at a high current density flows through a liquidconductor, then a strong electromagnetic pinch force acts thereon, withits contracting pressure being in proportion to a current density, sothat in the case of a MIG arc as shown in FIG. 15, a contractingpressure peaks at the tip of a welding wire of a diameter D_(o), whilethe contracting pressure is found not so high at the lower end D₂ (thetop surface of a crater). In case the internal pressure in a liquidconductor varies in this manner, then the liquid under a high pressureis necessarily forced towards a lower pressure portion, thus causing thetransferring of a liquid. As a result, in the case of an arc weldingusing a large current and high current density, a molten metal at thetip of a welding wire is splashed in the form of droplets into an arcspace at an gravitational acceleration of several tens G, whereuponthere is produced a strong air stream of a speed of over 1000 m/sec,which is directed from a high pressure portion of a small diametertowards a low pressure portion of a large diameter. This state, whereina molten metal is forcibly taken off the tip of a welding wire by suchan electromagnetic pinch force and then transferred to a base metal inthe form of fine particles, is referred to as a spray type transfer. Thespray type transfer uses a large current, so that a depositing rate ishigh and the penetration is deep, insuring highly efficient welding ofthick plates.

In case an arc current is lower than a given level, then a molten metalcan no longer be ejected only by an electromagnetic pinch force actingon a molten metal, from the tip of a welding wire. In other words, asurface tension force of a molten metal can no longer be negligiblerelative to the electromagnetic pinch force. In a current range lowerthan a given critical value, a molten metal produced at the tip of awelding wire assumes a round droplet form as shown in FIG. 16-A, whichin turn is grown to a certain size with the time lapse, and then droopsas shown in FIG. 16-B to contact a base metal. When the welding wire isbrought into contact with the base metal, then a greater part of adroplet is transferred to the side of the base metal according to atension force acting on the surface of a molten pool, as shown in FIG.16-C, while the remaining part of the molten metal assumes a thin lineor string form, as shown in FIG. 16-D. A large current flows through theaforesaid string-form portion, so that a strong electromagnetic pinchforce acts locally thereon and hence the string form portion is quicklynecked or broken, whereupon an arc is automatically produced again asshown in FIG. 16-E, returning to the state shown in FIG. 16-A. In thismanner, in a current range lower than a given critical value, therealternately takes place the production of an arc and shortcircuitingcyclically, and every time of the occurance of shortcoming, a droplet ofa small amount is shifted to the side of a base metal. The abovetransferring mode is referred to a short-circuiting type transfer. Inshort, during the production of an arc, the amount of a current flowingis less, so that an electromagnetic force acting on a droplet is not sogreat, while in the terminating phase of the shortcircuiting, theaforesaid electromagnetic force becomes cosiderably large, therebyinterrupting or breaking a stream of a molten metal to permit an arc tobe produced again, thus initiating a repeated cycle ofarc-shortcircuiting-arc. (30 to 150 cycles/sec.) Meanwhile, since an arcof a shortcircuiting type is produced in a small current range, and evenif a large circuiting current flows during a shortcircuiting durationcorresponding to half a welding duration, the heat is not generated byan arc, so that heat input to a base metal is less than in the case of aspray type arc, and hence the penetration into the base metal isshallow.

According to a pulsed arc welding, a large current is superposed oradded cyclically but for a short time on or to a base current as smallas a shortcircuiting current, thereby forcibly effecting spray transfer.The object of the pulse arc welding is to enable a spray arc mode evenin a low current range which could be obtained only in a shortcircuitingarc mode. FIG. 17 are diagrammatic views showing wave forms of weldingcurrents in a pulsed arc welding. In FIG. 17 (A), Ib represents asubstantially flat wave d.c. which may be obtained at a d.c. arc weldingsource of a general type, and is referred to as a basic current. Ip is apulsed current to be superposed on or added to the basic current, sothat a synthesized current I of the both currents serves as an arcdischarge current.

The value of a basic current Ib is not so large as to cause sprayshifing due to an electromagnetic pinch force produced by itself, ratherfalls in a shortcircuiting arc range, as compared with the diameter of awelding wire. However, when a pulsed current Ip is superposed on oradded to Ib, then a strong electromagnetic force acts on a molten metalfor a short time, whereupon the molten metal is forcible detached fromthe tip of a welding wire. An electromagnetic pinch force acting on aconductor of a given diameter is proportional to the square of a currentvalue, so that the maximum pinch force corresponding to a variation inthe current I as shown in FIG. 17 (A) is such as shown in FIG. 17 (B).When a time-integrated electromagnetic force exceeds a given value so asto allow a molten metal to overcome its surface tension, thereby beingdetached from the tip of a welding wire, then spray of one droplet isenabled. In case a peak current value is sufficiently large, then asshown in FIG. 17 (C), a droplet is ejected with a slight time delay froma current peak, and as the case may be, two or more droplets are ejectedfor one pulse. However, when a pulsed current is low, then as shown inFIG. 17 (D), there results a time lag until the droplet is ejected.

FIG. 18 shows a circuit diagram of one embodiment of a power sourcemeans for use in the present invention. According to this circuitarrangement, a basic d.c. current is supplied from a conventional typed.c. welding source, and then a pulsed current is added thereto from apulse generating source. Shown at Pd in FIG. 18 represents aconventional d.c. source, which is of a so-called d.c. constant voltagetype. Shown at Ps is a power source for generating pulses. In thisembodiment, a both-side wave rectifying bridge circuit including a pairof silicon rectifiers DR1, DR2 and a pair of thyristers SCR1, SCR2 isconnected to a single phase variable voltage type transformer T. Byadjusting a pulse signal generating circuit PG adapted to on-off controlgate signals from the thyristers SCR1, SCR2, a generating phase of amain pulse current or a width of a pulse current may be adjusted orpulse generating cycle per second may be set equal to or doubled ascompared with a power source frequency. The value of a wave front of apulse current may be adjusted by adjusting the secondary voltage of thetransformer T. P represents a welding wire, and M represents a weldingwire feeding device.

The value of pulse current and its generating cycle may be adjusted soas to be suited for an individual welding condition. However, it isrecommendable to use a generating cycle several times as high as a powersource frequency. In other words, it is recommendable to allow toprogressively switch between 50, 100 cycles per second for a 50 cyclepower source, and 60 and 120 cycles per second for 50 cycle powersource. In the actual welding operation, the shortcircuitingcycles/second of a shortcircuiting arc or the droplet generatingcycle/second of a spray arc falls in a range of 50 to 150 per second.Accordingly, better results may be achieved in a pulsed arc, when theforcible spray transfer is effected at cycles as above with pulsedcurrents.

FIG. 19 shows a circuit diagram of another embodiment of a power sourcefor use in the present invention. This circuit allows the generation ofa basic current and a pulse current from a single d.c. arc power source.In the circuit of this embodiment, a d.c. power source voltage isintentionally brought out of equilibrium, thereby generating a pulsecurrent. In FIG. 19, silicon rectifiers DR3 to DR8 are to form athree-phase all wave rectifying circuit. In this respect, a voltage ofone phase of an alternate current for DR3 is made variable by means of asliding mechanism, or made higher than a voltage of the other twophases, thereby rectifying the unbalanced three-phase voltages forobtaining a pulse current of a desired wave front value.

The following are the advantages of this embodiment: As has beendescribed earlier, according to the pulse welding, a large current issuperposed on a basic current as small as a shortcircuiting currentcyclically for a short time, thereby effecting a spray transferforcibly. The object is to enable a spray arc mode even in a lowercurrent range, which mode can not be obtained only in a shortcircuitingarc mode. The actual advantage of pulse arc welding lies in thepossibility of all position welding, such as vertical position welding,horizontal position welding, flat position welding and the like at ahigh speed, high efficiency and reliability. FIGS. 20A, 20B, 20C showtypical welding bead cross sectional configurations with reference to ashort-circuiting arc, pulsed arc and spray arc. The spray arc welding issuited for a welding operation at a large current and a high speed,providing a deep penetration, while short-circuiting arc welding is lessin penetration because of use of a small current, and subject to alimitation on a welding speed or depositing rate, thus resulting ininsufficient penetration, when the thickness of metals to be joined isincreased. On the other hand, in the case of all position welding suchas vertical position welding, horizontal position welding and the like,if a molten pool in arc welding is too large, then a molten metal tendsto drop. For this reason, even in the case of welding of metals of anincreased thickness, a current as small as 150 A should be used, whilethe current on this order falls in a short-circuiting arc range, andwould result in sufficient penetration. In contrast thereto, the pulsedarc welding permits a deep penetration for an average current in termsof the same level, and in addition a molten metal at the tip of awelding wire is shifted in a spray form to the side of a base metal,thus facilitating the movement of a wire, with the accompanying improvedefficiency in welding. As has been described earlier, in the case ofpulsed arc welding, a molten metal at the tip of a welding wire can bedetached off the tip of the wire by a force several times or severaltens times as high as the gravitational acceleration G, because of astrong electromagnetic pinch force produced due to a high currentdensity, so that it is of no importance whether a droplet is ejectedupwards against the gravity (overhead position welding) or drops in thedirection of gravity (flat position welding).

One of numerical examples is given as follows:

In the case of a thick plate butt-welding (a weld groove width is 8 mm,thickness of a plate is 200 mm, length of a plate is 6000 mm), a shieldgas consisting of argon and carbon dioxide gas of 80:20 is used at aflow rate of 75 litr/min. In the case of flat position welding, thewelding current used ranges from 240 to 260 A. In the case of horizontalposition welding, a welding current ranges from 140 to 180 A. In thesecases, a continuous stable welding may be carried out for each ofwelding positions.

The welding process according to this embodiment takes over thepositions of the prior art electro-slag welding and manual welding, andfind its wider application, because the narrow weld-groove MIG weldingprocess according to the present invention provides a stable weldingcondition, despite the weaving motion of a welding wire.

Further embodiment according to the present invention is shown in FIGS.21 to 29, which is intended to improve the defects in welds obtainedfrom a narrow weld-groove MIG welding according to the presentinvention.

FIG. 21 gives detailed observation of a molten pool and an arc in MIGwelding. A molten pool 92 is positioned under the wire 1, being somewhatbiased against the advancing direction of welding. With the progress ofwelding, there are formed beads 93 which are continuous, solidifiedlayers of metal, following a molten pool in the direction against theadvancing direction of welding. A molten metal assumes an elliptic formin cross section, because of a large surface-tension, as shown in FIG.21. A contacting angle θ formed by the surface of a bead and the surfaceof an under-layer 94 is larger than 90°, thus providing poorwettability. The example shown in FIG. 22 which is a cross-sectionalview taken along the line XIII--XIII of FIG. 21 provides poorwettability between the bead 93 and the metal 95, presenting a danger ofundercut and slag inclusion on a corner portion (a).

Hitherto, the supply of inert gas is effected through a passage 107around a contact tube 96, as shown in FIG. 21, or through a secondpassage 107a surrounding the passage 107 for sufficiently shielding aweld zone from atmosphere. The object of the supply of inert gas in theprior art welding is to shield a weld zone from air, rather than toaffect a molten pool, and thus is not intended to give some influence onthe molten pool intentionally.

According to this embodiment of the invention, there are provided aprocess and an apparatus therefor, in which inert gas is used not onlyfor shielding a weld zone from air but also for changing the contour ofa molten pool intentionally, thereby preventing defects in weldsobtained according to the narrow weld groove MIG, TIG welding processes.

This embodiment according to the present invention will be describedwith reference to MIG welding. FIG. 23 is a cross-sectional view of awelding apparatus, as viewed from the front in the advancing directionof welding, taken along the line XXIII--XXIII of FIG. 24. The transversecross section of the contact tube 96 is of an elongated rectangularcross section, and is formed with a hole 96a of a rectangular crosssection for accommodating a welding wire therethrough. The contact tube96 is high in electrical conductivity, while a welding wire which hasbeen waved according to the device described earlier is supplied fromabove through the hole 96a, continuously. Thus, beads are formed, withan arc produced at the lower end of the wire which is being weavedtransversely of a weld groove, and the welding wire is consumed,accordingly.

FIG. 24 is a cross-sectional veiw taken along the line XXIV--XXIV ofFIG. 23, in which a primary gas nozzle 97 is provided in close contactwith a contact tube forwardly in the advancing direction of welding. Aninert gas passage as shown in FIG. 24 is of an elongated rectangularcross section 97a as shown in FIG. 25, or is of a plurality of gaspassages provided in a block 97b as shown in FIG. 26. Alternatively, twoor more tubes may be joined together as shown in FIG. 27 in the blockform 97c. Cooling water tubes 98 are positioned in close contact withthe contact tube 96 in the rear thereof, as viewed in the advancingdirection of welding, while a secondary gas nozzle 99 is positioned infurther rear position of the cooling water tube 98 in close contacttherewith. The secondary gas nozzle should be of a shape similar to thatof the primary gas nozzle 97. In addition, the configuration of a gaspassages therefor may be either one of those shown in FIGS. 25 to 27.

Description will be had to a molten pool produced in a welding apparatusaccording to this embodiment. The longitudinal cross sectional built-upcontour of a molten pool in the prior art welding, in which a weld zoneis shield from air by inert gas, is shown by a one point chain line at100. In this embodiment, the secondary gas nozzle in a welding apparatusis positioned at a given spacing from the end of a welding wire, so thata molten pool 101 is spread under a gas pressure from the secondary gasnozzle, so that the surface of a molten pool assumes a concave crosssectional contour as shown at 100a. In addition, the primary gas nozzleis positioned in a given positional relationship to the end of a weldingwire, so that a molten metal, in a molten pool which has been forcedforwards by inert gas being injected from the secondary gas nozzle, ispushed back by the inert gas, thereby providing a concave crosssectional contour 100a, as shown in FIG. 28. Satisfactory penetration isachieved between a high temperature molten metal and an under-layer 102in contact therewith, and a sufficient penetration 104 is also achievedbetween a molten metal and a base metal, as shown at 100a in FIG. 28,being free of undercut and slag inclusion. In this manner, there may beformed beads 105 which are free of deflect, as the welding nozzle goeson. In addition, for the primary or secondary gas nozzle provided withtwo or more gas passages, as shown in FIG. 29, there may be provided gaspassages 106₁, 106₂, 106₃ which are adapted to deflect the direction ofgas being injected, thereby effectively pushing back the surface of amolten pool or spreading the molten pool. Alternatively, the flow ratesof gas through these gas passages may be varied, thereby facilitatingdeformation of a molten pool.

Assume that a distance of an axis of a contact tube to an axis of theprimary gas nozzle is "e", and a distance of the axis of the contacttube to the axis of the secondary gas nozzle is "f". Then, f>e. In otherwords, best results may be obtained, in case f is larger than e. Thecooling water tube 98 should not necessarily be limited to the instanceshown, but may be such as to surround the contact tube 96, so that thedistances f and e may be suitably selected. It is preferable that theprimary gas nozzle, contact tube, cooling water tube, and secondary gasnozzle be in intimate contact with each other for improving the heatradiation and the thermal conductivity thereof, in addition to reductionin size of the welding apparatus. This embodiment provides the followingfeatures:

(1) A narrow weld-groove welding process, in which the surface of amolten pool is spread by gas being injected through a secondary gasnozzle positioned in the rear of a contact tube as viewed in theadvancing direction of a contact tube, and a molten metal in a moltenpool is pushed back by gas being injected through a primary gas nozzlein front of the contact tube.

(2) A narrow weld-groove welding apparatus, in which one or more primarygas nozzles are positioned in front of a contact tube, while a coolingwater tube is positioned in the rear of the contact tube in intimatecontact therewith, and one or more secondary gas nozzles are positionedin the rear of the cooling water tube.

(3) A narrow weld-groove welding apparatus, in which there are providedtwo or more primary gas nozzles having varying injecting directions ofinert gas, and second gas nozzles having varying injecting directions ofinert gas.

(4) A narrow weld-groove welding apparatus, in which a distance of anaxis of a contact tube to an axis of the secondary gas nozzle is largerthan a distance of an axis of the contact tube to the axis of a primarygas nozzle.

According to the present invention, the surface of a molten pool may besuitably spread and pushed back, thereby providing a concave surface fora molten pool; sufficient or proper penetration may be achieved for abase metal and an under-layer; slag inclusion, undercut, blow holes maybe minimized; the size of a welding apparatus may be reduced; flatposition welding may be accomplished with ease; and horizontal positionwelding and other positions welding are also accomplished with ease,with the aid of a well retained molten pool due to the narrow weldgroove.

What is claimed is:
 1. A narrow weld-groove welding apparatus,comprising:a contact tube and means for advancing said contact tube;secondary gas supply means for spreading the surface of a molten metalin a molten pool laterally relative to an advancing direction of thecontact tube by gas being injected therethrough, said secondary gassupply means including a secondary gas nozzle means positioned in therear of the contact tube at a first distance from a center longitudinalaxis of the contact tube as viewed in the advancing direction of saidcontact tube and constructed for enabling a free flow of gas therefrom;and primary gas supply means for pushing back said molten metal in saidmolten pool relative to said advancing direction by gas being injectedtherethrough, said primary gas supply means including a nozzle meanspositioned in close proximity to the front of said contact tube at asecond distance from said center longitudinal axis of the contact tubeas viewed in said advancing direction, said second distance being lessthan said first distance, and said primary gas nozzle means beingconstructed for enabling a free flow of gas therefrom.
 2. A weldingapparatus as set forth in claim 1, wherein said apparatus furthercomprises:a cooling medium tube and its return tube for circulatingcooling medium therethrough and positioned adjacent to one side of acontact tube, said contact tube being adapted to guide and feed awelding wire into a weld groove; a shield gas supply tube positionedadjacent to the other side of said contact tube for supplying a shieldgas to said weld groove; and an impure-gas suction and discharge tubepositioned outwardly of said cooling-medium return tube and said shieldgas supply tube for introducing impure gas therein and discharging sameoutside; said tubes being all arranged flatwise in parallel relation toeach other, and the entire surfaces of said tubes being all covered withheat-resisting, electrically insulating material.
 3. A welding apparatusas set forth in claim 1, comprising a welding wire supply means forsupplying a welding wire to said contact tube, wherein said welding wiresupply means is provided with a rocking guide means which receives awelding wire from the direction which is substantially in parallel withthe axis of said roller means and supplies same to said roller means. 4.A welding apparatus as set forth in claim 1, comprising a welding wiresupply means for supplying a welding wire to said contact tube, whereinsaid welding wire supply means is provided with a welding-wire rockingguide means having a smooth passage which allows a deformation of saidwire within an elastic limit.
 5. A narrow weld groove welding apparatus,according to claim 1, wherein:said primary gas nozzle means comprisesone or more primary gas nozzles positioned in front of the contact tubein intimate contact therewith; a cooling medium tube is positioned inthe rear of said contact tube in intimate contact therewith; and saidsecondary gas nozzle means comprises one or more secondary gas nozzlespositioned in the rear of said cooling medium tube in intimate contacttherewith.
 6. A narrow weld-groove welding apparatus, as set forth inclaim 4, wherein said primary gas nozzle means comprises two or moreprimary gas nozzles having varying gas injecting directions fordirecting inert gas so as to produce said pushing back of the moltenmetal; and secondary nozzle means comprises gas nozzles having varyinginjecting directions for directing inert gas so as to produce saidlateral spreading of the molten metal.
 7. A narrow weld-groove weldingprocess, comprising the steps of:feeding a welding wire through acontact tube, forming a weld bead and advancing said contact tube, saidsteps further including the steps of: spreading the surface of a moltenmetal in a molten pool laterally relative to an advancing direction ofthe contact tube by gas being injected through a secondary gas nozzlepositioned in the rear of said contact tube at a first distance fromsaid welding wire as viewed in the advancing direction of said contacttube; and pushing back said molten metal in said molten pool relative tosaid advancing direction by gas being injected through a primary gasnozzle positioned in close proximity to the front of said contact tubeat a second distance from said welding wire, said second distance beingless than said first distance.
 8. A welding apparatus according to claim1, wherein said primary and secondary supply means are operable forsupplying inert gas to said primary and secondary nozzle means at variedpressures for producing said pushing back and lateral spreading of themolten metal, respectively.
 9. A welding method according to claim 7,wherein said steps of pushing back and spreading of the molten metal areperformed by directing said gas through said primary and secondarynozzle means in varied directions.
 10. A welding method according toclaim 7, wherein said steps of pushing back and spreading of the moltenmetal are performed by directing said gas through said primary andsecondary nozzle means at varied pressures.